Complete relief of left ventricular hypertension associated with cogenital aortic stenosis is often difficult to achieve employing conventional surgical methods. Aortic valvotomy, valve replacement or annuloplasty may not prove adequate particularly in patients with severe hypoplasia of the aortic annulus and malformed cusps. Subvalvar and supravalvar stenoses may also be present, which complicates surgical correction and often prevents elimination of the left ventricular-aortic pressure gradient. In many circumstances, the surgeon must adopt a limited objective directed toward partial relief of the obstruction without resorting to extensive valvuloplasty, which could lead to severe aortic regurgitation. In other patients, especially those with diffuse hypoplasia of the ascending aorta as well as the aortic valve annulus, no satisfactory form of corrective therapy is available. Recently, a patient with this form of aortic stenosis underwent implantation of a left ventricular-aortic bypass shunt which directed left ventricular output into the descending thoracic aorta. Following this procedure, the ventricular pressure was reduced to normal, and the patient has remained asymptomatic for the past 19 months. The present study is proposed to: (1) facilitate improvements in the design of the rigid apical portion of the composite bypass shunt; (2) evaluate the late competence of the upstented xenograft valves (in Dacron grafts) used in the shunt after prolonged exposure to aortic mean pressure; (3) investigate the biologic interface which forms within the device; (4) develop improved support for the tissue valve, especially at the commissural areas.